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Cap-binding protein (eukaryotic initiation factor 4E) and 4E-inactivating protein BP-1 independently regulate
1Department of Biochemistry, New York University Medical School, New York 10016, USA.
Abstract:
Cap-dependent protein synthesis in animal cells is inhibited by heat shock, serum deprivation, metaphase arrest, and infection with certain viruses such as adenovirus (Ad). At a mechanistic level, translation of capped mRNAs is inhibited by dephosphorylation of eukaryotic initiation factor 4E (eIF-4E) (cap-binding protein) and its physical sequestration with the translation repressor protein BP-1 (PHAS-I). Dephosphorylation of BP-I blocks cap-dependent translation by promoting sequestration of eIF-4E. Here we show that heat shock inhibits translation of capped mRNAs by simultaneously inducing dephosphorylation of eIF-4E and BP-1, suggesting that cells might coordinately regulate translation of capped mRNAs by impairing both the activity and the availability of eIF-4E. Like heat shock, late Ad infection is shown to induce dephosphorylation of eIF-4E. However, in contrast to heat shock, Ad also induces phosphorylation of BP-1 and release of eIF-4E. BP-1 and eIF-4E can therefore act on cap-dependent translation in either a mutually antagonistic or cooperative manner. Three sets of experiments further underscore this point: (i) rapamycin is shown to block phosphorylation of BP-1 without inhibiting dephosphorylation of eIF-4E induced by heat shock or Ad infection, (ii) eIF-4E is efficiently dephosphorylated during heat shock or Ad infection regardless of whether it is in a complex with BP-1, and (iii) BP-1 is associated with eIF-4E in vivo regardless of the state of eIF-4E phosphorylation. These and other studies establish that inhibition of cap-dependent translation does not obligatorily involve sequestration of eIF-4E by BP-1. Rather, translation is independently regulated by the phosphorylation states of eIF-4E and the 4E-binding protein, BP-1. In addition, these results demonstrate that BP-1 and eIF-4E can act either in concert or in opposition to independently regulate cap-dependent translation. We suggest that independent regulation of eIF-4E and BP-1 might finely regulate the efficiency of translation initiation or possibly control cap-dependent translation for fundamentally different purposes.
Insights
Heat shock and adenovirus inhibit cap-dependent translation by altering eukaryotic initiation factor 4E (eIF-4E) and 4E-binding protein (BP-1) phosphorylation. This regulation is independent, allowing for fine-tuning of protein synthesis.
Area of Science:
- Molecular Biology
- Cellular Biology
- Biochemistry
Background:
- Cap-dependent protein synthesis is crucial for animal cells but is inhibited by various stresses like heat shock and viral infections.
- This inhibition is mechanistically linked to the dephosphorylation and sequestration of eukaryotic initiation factor 4E (eIF-4E) by the repressor protein BP-1 (PHAS-I).
Purpose of the Study:
- To investigate the distinct roles of eIF-4E and BP-1 phosphorylation in regulating cap-dependent translation under stress conditions.
- To elucidate whether eIF-4E sequestration by BP-1 is a mandatory step for translation inhibition.
Main Methods:
- Analysis of eIF-4E and BP-1 phosphorylation and complex formation under heat shock and adenovirus infection.
- Experimental manipulation using rapamycin to assess the impact on phosphorylation pathways.
- In vivo association studies of eIF-4E and BP-1.
Main Results:
- Heat shock induces simultaneous dephosphorylation of both eIF-4E and BP-1, impairing translation.
- Adenovirus infection dephosphorylates eIF-4E but phosphorylates BP-1, leading to eIF-4E release.
- Inhibition of cap-dependent translation does not solely rely on eIF-4E sequestration by BP-1; independent regulation by their phosphorylation states is key.
Conclusions:
- Cap-dependent translation is independently regulated by the phosphorylation states of eIF-4E and BP-1.
- eIF-4E and BP-1 can act antagonistically or cooperatively to control translation efficiency.
- These findings suggest a sophisticated mechanism for fine-tuning protein synthesis initiation or serving distinct regulatory purposes.